This functionattempts to guess which of the several simple database modules
available — dbm.bsd, dbm.gnu, dbm.ndbm or
dbm.dumb — should be used to open a given file.

Returns one of the following values: None if the file can’t be opened
because it’s unreadable or doesn’t exist; the empty string ('') if the
file’s format can’t be guessed; or a string containing the required module
name, such as 'dbm.ndbm' or 'dbm.gnu'.

If the database file already exists, the whichdb() function is used to
determine its type and the appropriate module is used; if it does not exist,
the first module listed above that can be imported is used.

The optional flag argument can be 'r' to open an existing database for
reading only, 'w' to open an existing database for reading and writing,
'c' to create the database if it doesn’t exist, or 'n', which will
always create a new empty database. If not specified, the default value is
'r'.

The optional mode argument is the Unix mode of the file, used only when the
database has to be created. It defaults to octal 0o666 (and will be
modified by the prevailing umask).

The object returned by open() supports most of the same functionality as
dictionaries; keys and their corresponding values can be stored, retrieved, and
deleted, and the in operator and the keys() method are
available. Key and values are always stored as bytes. This means that when
strings are used they are implicitly converted to the default encoding before
being stored.

The following example records some hostnames and a corresponding title, and
then prints out the contents of the database:

importdbm# Open database, creating it if necessary.db=dbm.open('cache','c')# Record some valuesdb[b'hello']=b'there'db['www.python.org']='Python Website'db['www.cnn.com']='Cable News Network'# Note that the keys are considered bytes now.assertdb[b'www.python.org']==b'Python Website'# Notice how the value is now in bytes.assertdb['www.cnn.com']==b'Cable News Network'# Loop through contents. Other dictionary methods# such as .keys(), .values() also work.fork,vindb.iteritems():print(k,'\t',v)# Storing a non-string key or value will raise an exception (most# likely a TypeError).db['www.yahoo.com']=4# Close when done.db.close()

This module is quite similar to the dbm module, but uses the GNU library
gdbm instead to provide some additional functionality. Please note that the
file formats created by dbm.gnu and dbm.ndbm are incompatible.

The dbm.gnu module provides an interface to the GNU DBM library.
dbm.gnu.gdbm objects behave like mappings (dictionaries), except that keys and
values are always converted to bytes before storing. Printing a gdbm
object doesn’t print the
keys and values, and the items() and values() methods are not
supported.

It’s possible to loop over every key in the database using this method and the
nextkey() method. The traversal is ordered by gdbm‘s internal
hash values, and won’t be sorted by the key values. This method returns
the starting key.

If you have carried out a lot of deletions and would like to shrink the space
used by the gdbm file, this routine will reorganize the database. gdbm
objects will not shorten the length of a database file except by using this
reorganization; otherwise, deleted file space will be kept and reused as new
(key, value) pairs are added.

The dbm.ndbm module provides an interface to the Unix “(n)dbm” library.
Dbm objects behave like mappings (dictionaries), except that keys and values are
always stored as bytes. Printing a dbm object doesn’t print the keys and
values, and the items() and values() methods are not supported.

This module can be used with the “classic” ndbm interface, the BSD DB
compatibility interface, or the GNU GDBM compatibility interface. On Unix, the
configure script will attempt to locate the appropriate header file
to simplify building this module.

Open a dbm database and return a dbm object. The filename argument is the
name of the database file (without the .dir or .pag extensions;
note that the BSD DB implementation of the interface will append the extension
.db and only create one file).

The optional flag argument must be one of these values:

Value

Meaning

'r'

Open existing database for reading only
(default)

'w'

Open existing database for reading and
writing

'c'

Open database for reading and writing,
creating it if it doesn’t exist

'n'

Always create a new, empty database, open
for reading and writing

The optional mode argument is the Unix mode of the file, used only when the
database has to be created. It defaults to octal 0o666 (and will be
modified by the prevailing umask).

The dbm.dumb module is intended as a last resort fallback for the
dbm module when a more robust module is not available. The dbm.dumb
module is not written for speed and is not nearly as heavily used as the other
database modules.

The dbm.dumb module provides a persistent dictionary-like interface which
is written entirely in Python. Unlike other modules such as dbm.gnu no
external library is required. As with other persistent mappings, the keys and
values are always stored as bytes.

Open a dumbdbm database and return a dumbdbm object. The filename argument is
the basename of the database file (without any specific extensions). When a
dumbdbm database is created, files with .dat and .dir extensions
are created.

The optional flag argument is currently ignored; the database is always opened
for update, and will be created if it does not exist.

The optional mode argument is the Unix mode of the file, used only when the
database has to be created. It defaults to octal 0o666 (and will be modified
by the prevailing umask).

In addition to the methods provided by the collections.MutableMapping class,
dumbdbm objects provide the following method: